Reflecting device for electromagnetic waves

ABSTRACT

A device serves for reflecting electromagnetic waves in a length range less than 200 nm. It has a mirror carrier made of a material with at least approximately vanishing thermal expansion and at least one reflective layer applied on said mirror carrier. An intermediate layer made of a material which is formed such that its surface roughness is not significantly increased after beam processing methods is fitted between the mirror carrier and the reflective layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a reflecting device forelectromagnetic waves, comprising a mirror carrier made of a materialwith at least approximately vanishing thermal expansion and at least onereflective layer applied on said mirror carrier.

[0003] Moreover, the invention relates to a method for producing such adevice for reflecting electromagnetic waves.

[0004] More specifically, the invention relates to electromagnetic wavesin a wavelength range less than 200 nm.

[0005] 2. Description of the Related Art

[0006] DE 198 30 449 A1 discloses a mirror substrate which comprisescrystal and is provided with an amorphous layer of the order ofmagnitude of 1 to 100 μm, which allows a much higher polishing qualitythan the mirror substrate itself. In this case, the use of such a mirroris provided in EUV projection exposure installations.

[0007] In general, it is known that mirrors in EUV installations, forexample EUV lithography systems, must have a very good figure, whichmeans that the errors in the low spatial frequency range in EUV (spatialwavelengths≧1 mm) are small. Furthermore, such mirrors must have smallroughnesses in the mid spatial frequency range (MSFR: mid spatialfrequency roughness; at EUV spatial wavelengths typically between 1 μm⁻¹and 1 mm⁻¹. It is furthermore known that part of the incident light isabsorbed by the multilayer reflection layers that are customary andknown per se, the so-called Distributed Bragg Reflectors (DBR), andconverted into heat.

[0008] To ensure that the surface form of the mirror remains stableduring operation under these thermal loads, it is necessary to use amaterial with the smallest possible thermal expansion coefficient ascarrier material for such mirrors. In particular, mention shall be madehere of glass-ceramic materials which are composed of a plurality ofcomponents having different thermal expansion coefficients, so that thematerial has macroscopically no or a vanishing thermal expansioncoefficient. The materials sold under the proprietary names ZERODUR® orClear Ceram® are applicable as an example of said material.

[0009] In addition to these requirements that are to be satisfied in theregion of such mirrors, a very small roughness of approximately 0.1 nmrms of the surface must additionally be ensured in the region of highspatial frequencies (HSFR: high spatial frequency roughness; in EUV:spatial wavelengths≦1 μm), in order to ensure a correspondingreflectivity in the EUV region of the multilayer to be applied to themirror surface.

[0010] According to the current prior art, the required HSFR in theregion of approximately 0.1 nm rms can be achieved by means ofsuperpolishing methods on various materials, such as quartz glass, ULE,silicon, or else on glass ceramics, such as ZERODUR® or Clear Ceram®.However, these superpolishing methods have the disadvantage that, atleast when aspherical mirrors are used, generally the figures and, undercertain circumstances, also the longer-wave MSFR components are impairedagain, so that the superpolishing methods have to be followed by a finecorrection process.

[0011] Particularly when using glass-ceramic materials, such asZERODUR®, in which crystallites having a corresponding thermal expansioncoefficient are embedded in an amorphous matrix having a differentthermal expansion coefficient, a fine correction method, in particularbased on ion beam figuring, leads to a serious impairment of the HSFR.

SUMMARY OF THE INVENTION

[0012] Therefore, it is the object of the invention to provide a devicefor reflecting electromagnetic waves, in particular in a wavelengthrange less than 200 nm, which has a mirror carrier made of a materialwith at least approximately vanishing thermal expansion coefficient,wherein the abovementioned disadvantages with regard to the increase ofthe HSFR after a fine correction by means of beam processing methods areavoided.

[0013] According to the invention, this object is achieved by means of areflecting device, wherein an intermediate layer made of a materialwhich is formed such that its surface roughness is not significantlyincreased after beam processing methods is fitted between the mirrorcarrier and the reflective layer.

[0014] What can be achieved by virtue of the intermediate layer, whichretains the surface quality with regard to HSFR present before the beamprocessing after a use of beam processing methods, such as, for exampleIBF (Ion Beam Figuring), is that, in the case of a mirror carrier madeof a material with approximately vanishing thermal expansion, acorrection by means of the beam processing methods, which operate veryaccurately and controllably, is made possible without the surface of themirror carrier being impaired in the process before the application ofthe reflective layer to an extent such that a loss of reflectivity needbe feared.

[0015] This is because the inventors have found that during theprocessing of such materials with vanishing thermal expansion, which,according to the currently known prior art, predominantly comprise, atleast in the microscopic region, two or multiphase mixtures, theindividual constituents are removed to different degrees by beamprocessing methods, so that the surface roughness achieved beforehand bymeans of superpolishing methods is impaired again after the beamprocessing.

[0016] By contrast, the intermediate layer made of a homogeneousmaterial permits the beam processing without losses of quality withregard to HSFR, so that corrections in the context of the layerthickness of the intermediate layer are possible without any problemsand without, in the process, impairing the surface roughness of themirror carrier itself.

[0017] In particularly expedient refinements of the invention, at lightwavelengths in the visible region, the intermediate layer comprisesreflective material, in particular silicon.

[0018] In an intermediate layer made of silicon, what is additionallyachieved, as shown in experiments, is that the surface quality withregard to HSFR can be improved again relative to the surface quality ofthe original surface, lying below the intermediate layer, by means ofthe beam processing of silicon. It is thus possible to achieve HSFRqualities which lie significantly below 0.1 nm rms. Such devices forreflecting electromagnetic waves are thus highly suitable even forelectromagnetic waves having a wavelength in the range from 10 to 20 nm,in connection with the multi-layer reflection layers known per se, forachieving a very high reflectivity.

[0019] A method for producing a device which satisfies the objectmentioned above is defined in greater detail by the characterizing partof claim 9.

[0020] In the method, it is provided that, in a first step, surfaceroughnesses which are less than or at least equal to 0.25 to 0.1 nm rmsare realized by means of polishing or superpolishing methods known perse. In the next method step, the corresponding intermediate layer isthen applied to the superpolished mirror carrier.

[0021] Since it is the case with superpolishing methods that smallerrors can very often occur in the region of the figure or of thelonger-wave MSFR, these errors are corrected by means of beam processingmethods on the surface form of the mirror carrier in the intermediatelayer. In this case, with regard to its layer thickness, thisintermediate layer must be configured such that the surface of theactual mirror carrier is not concomitantly processed during the beamprocessing methods.

[0022] In a final method step, a reflective layer, in particular as amultilayer layer known per se, is applied to the mirror carrier surfacethat has been processed in this way and satisfies the correspondingrequirements with regard to the surface form, the figure and also MSFRand HSFR.

[0023] Consequently, for the first time one is able to use beamprocessing methods for effectively influencing the surface form withregard to figure and MSFR in the case of mirror carriers with vanishingthermal expansion, which usually comprise a glass ceramic with amultiphase mixture at least in the microscopic region.

[0024] Further advantageous refinements of the invention emerge from theremaining subclaims and the exemplary embodiment which is illustratedbelow with reference to the drawing.

DETAILED DESCRIPTION

[0025] The single accompanying figure shows a diagrammatic cross sectionthrough part of a mirror carrier with coating shown in a greatlyexaggerated illustration.

[0026] The detail shows part of a mirror carrier 1 which is shown in abasic illustration with an intermediate layer 2 shown in a greatlyexaggerated illustration and a reflective layer 3 formed, in particular,as a multilayer layer known per se (Distributed Bragg Reflectors/DBR).In order to meet stringent requirements with regard to the thermalstability, for example the requirements in EUV lithography objectives,the mirror carrier 1 must be formed from a material having an at leastapproximately vanishing thermal expansion, in order to remainuninfluenced, with regard to the imaging quality, by instances ofheating which are unavoidable on account of radiation absorbed by themultilayer 3.

[0027] In addition to these requirements with regard to the thermalexpansion, the mirror carrier 1 must have, at its surface 4 which latercarries the multilayer layer 3, very stringent requirements with regardto the figure, which is responsible for the imaging quality, with regardto the MSFR (mid spatial frequency roughness), which is responsible forscattering effects and contrast, and with regard to the HSFR (highspatial frequency roughness), which is responsible for the reflectivity.In order to be able to operate in the range of wavelengths below 200 nm,for example with X-ray waves in the range of λ=10-20 nm, for example thevalue of the HSFR must be significantly less than 0.5 nm rms, preferably0.2 nm rms, particularly preferably 0.1 nm rms.

[0028] Conventional superpolishing methods are perfectly capable ofobtaining such surface qualities with regard to HSFR. In the case ofmirrors, in particular in the case of aspherical mirrors, the figure andlonger-wave ranges of the MSFR suffer, however, as a result of thesesuperpolishing methods.

[0029] It seems reasonable to attempt to correct these errors in theregion of the figure and the longer-wave MSFR by means of beamprocessing methods, in particular by means of IBF (Ion Beam Figuring),since these methods are already used in other areas of optics forsimilar corrections of the figure.

[0030] It has been shown, however, that a serious impairment of the HSFRoccurs in the case of the materials that are used for the mirror carrier1 and have at least approximately vanishing thermal expansion, duringthe beam processing.

[0031] The materials appropriate for the mirror carrier 1 arepredominantly glass ceramics or other materials which are built up, atleast in the microscopic region, from different phases, with differentthermal expansion in each case. These different phases or differentmaterials react to different degrees, however, that is to say with aremoval rate of different magnitude, to the processing by means of IBFor comparable beam processing methods.

[0032] By way of example, in the case of the material sold under theproprietary name ZERODUR, which comprises a glass matrix withcrystallites embedded therein, it is shown that, by means of IBF, thecrystallites, which have a size of approximately 50 nm, are “preparedout” from the material surrounding them. The surface quality of thesurface 4 with regard to HSFR is thereby seriously impaired.

[0033] This problem can be solved by the intermediate layer 2. To thatend, the surface 4 is superpolished in a manner known per se by means ofsuperpolishing methods to the corresponding surface requirements, forexample HSFR=0.1 nm rms. This is followed by the application of theintermediate layer 2, which is applied in a comparatively thin layerthickness, for example a layer thickness of between 100 nm and 10 μm, sothat the thermal expansion of the intermediate layer is negligible incomparison with the mirror substrate.

[0034] Customary coating methods can be used as method for applying theintermediate layer, sputtering having proved to be particularly suitableand readily manageable with regard to the process control, which isagain responsible for the imaging of the quality of the surface 4 in thesurface 5.

[0035] Using beam processing methods, it is now possible to effect acorrection with regard to figure and longer-wave MSFR of theintermediate layer 2, so that a surface 5 of the intermediate layer 2,after this processing method, satisfies all the specifications andrequirements with regard to thermal expansion, figure, MSFR and HSFR.

[0036] The multilayer 3 known per se can then be applied as reflectionlayer to said surface 5 of the intermediate layer 2.

[0037] The material properties of the intermediate layer 2 must beselected such that the material of the intermediate layer reacts to beamprocessing methods by very uniform removal. To that end, theintermediate layer 2 may comprise, for example, silicon-containingmaterials such as quartz glass or the like. The use of silicon itself ormetals is also conceivable. On account of the requirement that thefigure of the surface 5 be measurable by means of interferometers,opaque materials are certainly preferable since they cause no disturbinginterference from their layer thickness and, consequently, can be betterdetected by interferometric measurement methods.

[0038] Particularly expedient results, as have been shown, are producedwhen silicon is used for the intermediate layer 2. On account of itshighly homogeneous construction, silicon reacts very positively to beamprocessing methods, in particular to IBF. The HSFR of IBF-processedsilicon layers can additionally be increased relative to the HSFR of thelayer provided below the silicon layer by means of the IBF processing,so that, when a silicon layer is used, it is possible to obtain afurther improvement in the surface 5 relative to the surface 4 withregard to HSFR.

[0039] Thus, the intermediate layer 2 constitutes a layer whichconserves the HSFR of the surface 4 or a layer which even improves itfurther when silicon is used, as in the manner mentioned above.

[0040] In principle, however, even with the use of layers which coarselyretain the HSFR or impair it at least only very slightly after theprocessing by IBF, a further superpolishing method on the intermediatelayer 2 could be used to obtain the desired quality of the surface 5.

What is claimed is:
 1. A reflecting device for electromagnetic waves,comprising a mirror carrier made of a material with at leastapproximately vanishing thermal expansion and at least one reflectivelayer applied on said mirror carrier, wherein an intermediate layer madeof a material which is formed such that its surface roughness is notsignificantly increased after beam processing methods is fitted betweenthe mirror carrier and the reflective layer.
 2. The device as claimed inclaim 1, wherein the electromagnetic waves are in a wavelength rangeless than 200 nm.
 3. The device as claimed in claim 1, wherein saidmirror carrier comprises a glass-ceramic material with embeddedcrystallites.
 4. The device as claimed in claim 1, wherein the thicknessof said intermediate layer lies between 100 nm and 10 μm.
 5. The deviceas claimed in claim 1, wherein said intermediate layer comprises asilicon-containing material.
 6. The device as claimed in claim 5,wherein said intermediate layer comprises silicon.
 7. The device asclaimed in claim 1, wherein said intermediate layer comprises quartzglass.
 8. The device as claimed in claim 1, wherein said intermediatelayer comprises metal.
 9. A method for producing a device for reflectingelectromagnetic waves, comprising a mirror carrier made of a materialwith at least approximately vanishing thermal expansion and at least onereflective layer applied on said mirror carrier, wherein in a firststep, the mirror carrier is superpolished to surface roughnesses in thespatial frequency range less than/equal to 1 μm⁻¹ less than/ equal to0.25 nm rms; in a second step, an intermediate layer is applied to thesuperpolished mirror carrier, in a third step, a correction of thesurface form of the mirror carrier is carried out by means of beamprocessing methods and, in a fourth step, the reflective layer isapplied to the mirror carrier.
 10. The method as claimed in claim 9,wherein the electromagnetic waves are in a wavelength range less than200 nm.
 11. The method as claimed in claim 9, wherein as reflectivelayer a multilayer layer is used.
 12. The method as claimed in claim 9,wherein a renewed polishing is carried out at least once between thethird and fourth steps.
 13. The method as claimed in claim 9, whereinIon Beam Figuring is used as the beam processing method.
 14. The methodas claimed in claim 9, wherein said intermediate layer is applied tosaid mirror carrier by means of sputtering.
 15. The method as claimed inclaim 9, wherein said intermediate layer is applied to said mirrorcarrier by means of electron beam evaporation.
 16. The use of the deviceas claimed in claim 1 in an objective for EUV lithography.
 17. The useof the device as claimed in claim 1 in a reflection mask for EUVlithography.
 18. The use of the device as claimed in claim 1 in anobjective for EUV microscopy.
 19. The use of the device as claimed inclaim 1 in an objective for EUV astronomy.